tustvold commented on code in PR #6178:
URL: https://github.com/apache/arrow-rs/pull/6178#discussion_r1709440874
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arrow-buffer/src/buffer/mutable.rs:
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@@ -668,6 +668,56 @@ impl MutableBuffer {
}
}
+impl<'a> MutableBuffer {
+ /// Creates a [`MutableBuffer`] from an `&[u8]` [`Iterator`] with a
trusted (upper) length.
+ /// Prefer this to `collect` whenever possible, as it is faster.
+ /// # Panic
+ /// This method will panic if `&u8.len()` != `item_size`.
+ /// # Example
+ /// ```
+ /// # use arrow_buffer::MutableBuffer;
+ /// # use arrow_buffer::ToByteSlice;
+ /// let iter = vec![[1_u8, 2].to_byte_slice(), [3_u8,
4].to_byte_slice()].into_iter();
+ /// let buf = unsafe {
+ /// MutableBuffer::from_trusted_len_iter_slice_u8(iter, 2)
+ /// };
+ /// assert_eq!(4, buf.len());
+ /// ```
+ /// # Safety
+ /// This method assumes that the iterator's size is correct and is
undefined behavior
+ /// to use it on an iterator that reports an incorrect length.
+ // This implementation is required for two reasons:
+ // 1. there is no trait `TrustedLen` in stable rust and therefore
+ // we can't specialize `extend` for `TrustedLen` like `Vec` does.
+ // 2. `from_trusted_len_iter_slice_u8` is faster.
+ #[inline]
+ pub unsafe fn from_trusted_len_iter_slice_u8<I: Iterator<Item = &'a [u8]>>(
Review Comment:
Right from_trusted_len_iter and from_trusted_len_iter_bool make sense
because they're using primitives with a fixed width known at compile time, this
allows for effective vectorisation. With variable width types, as in
from_trusted_len_iter_slice_u8, the compiler will just insert a memcpy for each
element, and there will be no performance benefit
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